Process for dehumidifying moist gas mixtures

ABSTRACT

The invention relates to a process for dehumidifying a moist gas mixture and to the absorption medium used in the process. The invention further relates to an apparatus for dehumidifying a moist gas mixture and to the use of said apparatus in the process according to the invention.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority under 35 USC § 119 to European application, EP 16174299.4, filed on Jun. 14, 2016, the contents of which is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The invention relates to a process for dehumidifying a moist gas mixture and to the absorption medium used in the process. The invention further relates to an apparatus for dehumidifying a moist gas mixture and to the use of said apparatus in the process according to the invention.

BACKGROUND OF THE INVENTION

Dehumidification of moist gas mixtures is necessary in a multitude of technical fields. For instance ventilation and air conditioning of buildings or vehicles generally necessitates not only cooling but also dehumidification of the air since the air to be cooled is often so humid that during cooling to the desired temperature the temperature falls below the dew point temperature. Hence in conventional air conditioning systems dehumidification of the air accounts for a large part of the electricity consumption.

The electricity consumption of air conditioning systems for buildings is reduced by dehumidifying the air by adsorption or absorption of water with a drying medium and subsequently regenerating the water-laden drying medium by heating to a temperature at which the water is desorbed again. Compared to adsorption on a solid absorbent, the advantage of absorption in a liquid absorption medium is that air dehumidification can be carried out with reduced equipment complexity and with less drying medium and that regeneration of the water-laden drying medium using solar heat is easier to carry out.

A further technical field where dehumidifying of moist gas mixtures is employed is the field of absorption chillers (synonymous with “absorption heat pump”, principle described in WO 2014/079675 A1). Here, the damp gas mixture is formed during evaporation of water under low pressure. The water vapour thus formed needs to be removed from the moist gas mixture so that said mixture may then be returned to the water evaporation to pass through a new cycle. Here too, absorption in a liquid absorption medium is favoured over adsorption on a solid adsorption medium.

To absorb moisture from ionic liquids the prior art suggests a series of ionic liquids. Thus, DE 10 2010 004 779 A1 mentions 1-ethyl-3-methylimidazolium ethylsulfate, 1-ethyl-3-methylimidazolium methylsulfate, 1-ethyl-3-methylimidazolium diethylphosphate, 1-ethyl-3-methylimidazolium methylsulfonate, 1-butyl-3-methylimidazolium bistrifluoromethanesulfonimide, 1-butyl-3-ethylimidazolium chloride for example.

CN 102335545 A describes a series of ionic liquids based on alkyl phosphates, namely inter alia 1,3-dimethyimidazolium dimethylphosphate, 1-ethyl-3-methylimidazolium dimethylphosphate and 1-butyl-3-methylimidazolium dimethylphosphate.

Y. Luo et al., Appl. Thermal Eng. 31 (2011) 2772-2777 proposes using the ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate in place of an aqueous solution of lithium bromide for air dehumidification. However, this ionic liquid has the disadvantage of only poor absorption capability.

These and further ionic liquids are also discussed by Krannich et al., J. Chem. Eng. Data 61 (2016), 1162-1176.

Y. Luo et al., Solar Energy 86 (2012) 2718-2724 proposes the ionic liquid 1,3-dimethyimidazolium acetate as an alternative to 1-ethyl-3-methylimidazolium tetrafluoroborate for air dehumidification. The acetates are also discussed by M. Kanakubo et al., J. Mol. Liq. 217 (2016) 112-119.

US 2011/0247494 A1 proposes a further ionic liquid for this purpose, namely 1-ethyl-3-methylimidazolium acetate.

The application of these ionic liquids in absorption chillers is also discussed in WO 2013/050242 A1.

One important criterion for choosing an ionic liquid as absorption medium is the viscosity thereof. It is thus desirable to employ an absorption medium having the lowest possible viscosity since easier flowability of the absorption medium markedly simplifies the use thereof in air dehumidification. In an absorption chiller, in which the absorption medium needs to pumped, a more flowable absorption medium can be transported with reduced energy consumption which markedly enhances the economy of such a chiller. In addition, a lower viscosity also results in fewer outages caused for instance by cavity blockages.

Such blockages also have a further cause: It was observed that many ionic liquids described in the literature show a propensity to precipitate on contact with CO₂, thus forming solid deposits. However, the presence of CO₂ in the gas streams for dehumidification is in many cases unavoidable and in specific applications, for example dehumidification of natural gas, the proportion of CO₂ in the gas stream for dehumidification is very high.

It is accordingly an object of the present inventions to provide an absorption medium which does not exhibit the abovementioned problems, i.e. has the lowest possible viscosity and a low propensity to precipitate on contact with CO₂.

An absorption medium which achieves this object has now surprisingly been found.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1: FIG. 1 illustrates an embodiment of the apparatus V2/V1 according to the invention.

FIG. 2; FIG. 2 illustrates in schematic fashion an absorption chiller into which an apparatus V2 is integrated.

DETAILED DESCRIPTION OF THE INVENTION

The present invention accordingly relates to a process for dehumidifing a moist gas mixture G₁, wherein the moist gas mixture G₁ is contacted with a liquid absorption medium A₁ which comprises a mixture of triethylene glycol and at least one salt S selected from the group consisting of Q⁺A⁻, Q⁺(R¹O)₂PO₂ ⁻, (Q⁺)₂R²OPO₃ ²⁻, Q⁺M⁺R³OPO₃ ²⁻ to obtain a gas mixture G₂ which has a lower water content compared to moist gas mixture G₁ and a liquid absorption medium A₂ which has an elevated water content compared to the liquid absorption medium A₁, wherein Q⁺ is a dialkylimidazolium cation in which the alkyl groups each independently of one another have 1 to 6 carbon atoms, wherein A⁻ is an anion selected from the group consisting of acetate, propionate, R′SO₃ ⁻, HSO₄ ⁻, R″SO₄ ⁻, wherein R′, R″ are each independently of one another an alkyl group having 1 to 6 carbon atoms, wherein R¹, R², R³ are each independently of one another an alkyl group having 1 to 6 carbon atoms and M⁺ is an alkali metal ion, with the proviso that the salt S is not dimethylimidazolium methylsulfonate. Triethylene glycol has CAS number 112-27-6.

“Moist” is to be understood as meaning in the context of the invention “comprising water, in particular water vapour”. “Dehumidifying” is to be understood as meaning in accordance with the invention “at least partially removing water”.

“At least partially” is to be understood as meaning in the context of the invention “partially or completely”.

“Moist gas mixture G₁” is accordingly to be understood as meaning in the context of the invention that the gas mixture G₁ comprises water, preferably water vapour (“water vapour” is to be understood as meaning water in the gaseous physical state), and that its composition is otherwise not particularly restricted. The water content of the moist gas mixture is not particularly restricted and is in particular from 0.01 vol % to 99.99 vol % (“vol %” indicates the volume of water vapour based on the overall volume of moist gas mixture G₁). The composition of the moist gas G₁ may otherwise vary depending on the application of the process according to the invention. The moist gas mixture G₁ is in particular selected from moist natural gas, moist air (this may be moist indoor air or the moist air resulting from evaporation of water in absorption chillers), preferably moist air. For moist natural gas the water content is in particular from 0.01 vol % to 15.00 vol %, for moist air said content is in particular from 0.01 vol % to 5.00 vol % in the case of moist indoor air or in particular from 95.00 vol % to 99.99 vol %, which is the preferred range, when moist air resulting from evaporation of water in absorption chillers is concerned.

The gas mixture G₁ in particular comprises CO₂. The gas mixture G₁ preferably comprises CO₂ in a proportion of 1 to 140000 ppm, more preferably 100 ppm to 20000 ppm, yet more preferably 350 ppm to 10000 ppm.

The proportion of CO₂ in the gas mixture G₁ may be determined by those skilled in the art with commercially available devices (CO₂ sensors) or IR spectroscopy for example as a matter of routine.

The process according to the invention is in particular carried out in an apparatus V₁. The apparatus V₁ is restricted only to the extent that it shall be suitable for carrying out the process according to the invention. In particular, an apparatus V₁ having the following components may be employed:

-   -   (i) at least one water absorption unit W_(abs1) set up for         contacting the moist gas mixture G₁ with the liquid absorption         medium, in particular A₁,     -   (ii) at least one water desorption unit W_(des1) which comprises         a heat exchanger W_(x1) and which is set up for at least         partially removing water from a liquid absorption medium, in         particular A₂,     -   (iii) and a circuit U₁, which connects the water absorption unit         W_(abs1) with the water desorption unit W_(des1) and by means of         which the liquid absorption medium, in particular A₁/A₂, may be         circulated.

The water absorption unit W_(abs1) in particular carries out the process according to the invention. Employable water absorption units W_(abs1) include in particular the water absorbers known to those skilled in the art. Said absorbers are based on the principle of increasing the surface area of the liquid absorption medium, in particular A₁, and simultaneously achieving the longest possible residence time of the liquid absorption medium A₁ in the water absorber during absorption of water. It is, in particular, possible here to employ water absorbers selected from the group of: packed beds, spray columns, falling-films, bubble columns, tray columns, wet scrubbers (for example Venturi scrubbers), stirred tanks and combinations of these absorbers. It is particularly preferable to employ falling-films, in particular shell and tube falling-films, as water absorbers. The water absorption unit W_(abs1) may in particular also comprise an additional heat exchanger W_(z1) set up such that the liquid absorption medium, in particular A₁ is coolable.

The water desorption unit W_(des1), which comprises a heat exchanger W_(x1) in particular carries out an optional step of the process according to the invention [“step b)”, wherein in this context the process would be step a)]. The water desorption unit W_(des1) is based on the principle of supplying heat to the water-laden liquid absorption medium A₁, increasing the surface area of the water-laden liquid absorption medium A₁ and simultaneously achieving the longest possible residence time of the water-laden liquid absorption medium A₂ in the water desorption unit W_(des1).

Employable water desorption units W_(des1) comprising a heat exchanger W_(x1) include in particular the combinations of heat exchanger and water desorber known to those skilled in the art, in particular horizontal tube evaporators having an upstream heat exchanger, in particular shell and tube heat exchangers, plate and frame heat exchangers. In addition the water desorption unit W_(des1) comprising a heat exchanger W_(x1) may also be a water desorber having an integrated heat exchanger. Such water desorbers having an integrated heat exchanger are in particular climbing film evaporators, long tube vertical evaporators, short tube vertical evaporators, forced circulation evaporators, agitated thin film evaporators. It is particularly preferable to employ falling-films, in particular shell and tube falling-films, as water desorption unit W_(des1).

Circuit U₁ in particular passes A₂ from step a) of the process according to the invention from the water absorption unit W_(abs1) to the water desorption unit W_(des1) and more preferably—in particular when the process according to the invention is carried out in continuous fashion—additionally passes A₃ from step b) of the process according to the invention from the water desorption unit W_(des1) to the water absorption unit W_(abs1).

The circuit U₁ is in particular a conduit, in particular selected from the group consisting of tube, hose.

In a further preferred embodiment the circuit U₁ also comprises a pump.

The contacting may be effected in any way known to those skilled in the art, in particular in a water absorption unit W_(abs1). The contacting causes the absorption medium A₁ to at least partially absorb moisture, i.e. water, from the moist gas stream G₁.

It is preferable to cool the absorption medium A₁ during contacting of the moist gas mixture G₁ in order that as much moisture as possible is absorbed from the moist gas mixture G₁. This may be achieved, for example, via an additional heat exchanger W_(z1) in the water absorption unit W_(abs1). The temperature of the absorption medium A₁ during contacting of the moist gas mixture G is thus preferably in the range from 15° C. to 90° C., more preferably from 20° C. to 80° C., most preferably from 20° C. to 40° C.

In the process according to the invention the moist gas mixture G₁ is contacted with a liquid absorption medium A₁ which comprises a mixture of triethylene glycol and at least one salt S selected from the group consisting of Q+A⁻, Q⁺(R¹O)₂PO₂ ⁻, (Q⁺)₂R²OPO₃ ²⁻, Q⁺M⁺R³OPO₃ ²⁻. Here, Q⁺ is a dialkylimidazolium cation in which the alkyl groups each independently of one another have 1 to 6 carbon atoms and A⁻ is an anion selected from the group consisting of acetate, propionate, R′SO₃ ⁻, HSO₄ ⁻, R″SO₄ ⁻, wherein R′, R″ are independently of one another an alkyl group having 1 to 6 carbon atoms, wherein R¹, R², R³ are each independently of one another an alkyl group having 1 to 6 carbon atoms and M⁺ is an alkali metal ion, preferably Li⁺, K⁺ or Na⁺, yet more preferably K⁺ or Na⁺. However, according to the invention the salt S is not dimethylimidazolium methylsulfonate (which is of course also the case in the preferred embodiments of the invention).

In a preferred embodiment of the process according to the invention Q⁺ is a dialkylimidazolium cation in which the alkyl groups each independently of one another have 1 to 4 carbon atoms and R¹, R², R³ are each independently of one another methyl or ethyl and A⁻ is selected from the group consisting of acetate, propionate R′SO₃ ⁻, HSO₄ ⁻, R″SO₄ ⁻, wherein R′, R″ are independently of one another methyl or ethyl.

In a more preferred embodiment of the process according to the invention the salt S is selected from the group consisting of Q⁺A⁻, Q⁺(R¹O)₂PO₂ ⁻ and Q⁺ is a dialkylimidazolium cation in which the alkyl groups are each independently of one another methyl, ethyl, n-propyl, iso-propyl or n-butyl and A⁻ is an anion selected from the group consisting of acetate, propionate, R′SO₃ ⁻, R″SO₄ ⁻, wherein R′, R″ are independently of one another an alkyl group having 1 to 4 carbon atoms, wherein R¹ is methyl or ethyl.

In a more preferred embodiment of the process according to the invention the salt S is selected from the group consisting of Q⁺A⁻, Q⁺(R¹O)₂PO₂ ⁻ and Q⁺ is a dialkylimidazolium cation in which the alkyl groups are each independently of one another methyl, ethyl, n-propyl, iso-propyl or n-butyl and A⁻ is an anion selected from the group consisting of acetate, propionate, R′SO₃ ⁻, R″SO₄ ⁻, wherein R′, R″ are independently of one another an alkyl group having 1 to 2 carbon atoms, wherein R¹ is methyl or ethyl.

In a yet more preferred embodiment of the process according to the invention the salt S is selected from the group consisting of Q⁺A⁻, Q⁺(R¹O)₂PO₂ ⁻ and Q⁺ is selected from the group consisting of 1,3-dimethylimidazolium cation, 1,3-diethylimidazolium cation, 1-ethyl-3-methylimidazolium cation and A⁻ is an anion selected from the group consisting of acetate, propionate, MeSO₃ ⁻, EtSO₃ ⁻, MeSO₄ ⁻, EtSO₄ ⁻ and R¹ is methyl or ethyl.

In a yet still more preferred embodiment of the process according to the invention the salt S is selected from the group consisting of Q⁺A⁻, Q⁺(R¹O)₂PO₂ ⁻ and Q⁺ is selected from the group consisting of 1,3-dimethylimidazolium cation, 1,3-diethylimidazolium cation, 1-ethyl-3-methylimidazolium cation and A⁻ is an anion selected from the group consisting of acetate, propionate, MeSO₃ ⁻, EtSO₄ ⁻ and R¹ is methyl or ethyl, yet more preferably R¹=ethyl.

In a particularly preferred embodiment of the process according to the invention Q⁺ is selected from the group consisting of 1,3-dimethylimidazolium cation, 1,3-diethylimidazolium cation, 1-ethyl-3-methylimidazolium cation and A⁻ is an anion selected from the group consisting of acetate, propionate. It is yet still more preferable if Q⁺ is then a 1,3-dimethylimidazolium cation.

This is because in addition to the previously discussed effect yet another positive effect of the process according to the invention is observed in the case of the acetates and propionates: When CO₂ is present in the moist gas stream acetates and propionates precipitate which results in undesired deposits in the apparatus. It has now been found that this propensity is negated for these salts S when they are present in a mixture with triethylene glycol. This observation was likewise entirely surprising.

The liquid absorption medium A₁ may be employed in the process according to the invention in the form of the pure mixture of triethylene glycol and the salts S. Alternatively and more preferably the liquid absorption medium A₁ is an aqueous solution. It is preferable when in said aqueous solution the total weight of triethylene glycol and all salts S is at least 70 wt % based on the total weight of the aqueous solution. In said aqueous solution the total weight of triethylene glycol and all salts S is yet more preferably at least 80 wt % based on the total weight of the aqueous solution, yet more preferably 85 wt %, yet more preferably 90 wt %, yet still more preferably 99 wt %.

In the process according to the invention the ratio of triethylene glycol to the salts S in the absorption medium A₁ is not further restricted. However, it is preferable to employ in the process according to the invention an absorption medium A₁ in which the ratio of the total weight of the triethylene glycol to the total weight of all salts S is in the range 1:9 to 9:1, more preferably 2:8 to 8:2, yet more preferably 3:7 to 7:3, yet still more preferably 3:7 to 6:4, yet still more preferably 3:7 to 1:1, most preferably 3:7 to 4:6, with very greatest preference 3:7.

The gas mixture G₂ obtained in the process according to the invention and having a relatively low water content compared to the moist gas mixture G₁ then represents the dehumidified gas stream which, depending on the application, can be returned to living or working spaces in the form of dehumidified air or in the case of natural gas can be supplied to power generation.

The liquid absorption medium A₂ obtained in the process according to the invention has an elevated water content compared to the liquid absorption medium A_(1*) it will be appreciated that in terms of the salts S comprised by it and in terms of the triethylene glycol comprised by it A₂ is identical to A₁ and is preferably distinguished therefrom only by its water content.

An optional additional step of the process according to the invention [“optional step b)”, wherein the process according to the invention is to be understood as “step a)”] then effects the at least partial removal of water from the liquid absorption medium A₂ to obtain a liquid absorption medium A₃ having a relatively low water content compared to the liquid absorption medium A₂. Heat is additionally supplied to the liquid absorption medium A₂ here. The supply of heat and the at least partial removal may be effected in any way known to those skilled in the art, in particular in a water desorption unit W_(des1) comprising a heat exchanger W_(x1). The at least partial removal of water from the liquid absorption medium A₂ affords a liquid absorption medium A₃ having a relatively low water content compared to the liquid absorption medium A₂.

It will be appreciated that in terms of the salts S comprised by it and in terms of the triethylene glycol comprised by it A₃ is identical to the A₂ and is preferably distinguished therefrom only by its water content.

In a further preferred embodiment the process according to the invention is carried out in continuous fashion. This means in particular that following step b) steps a) and b) are performed at least one further time and the liquid absorption medium A₁ employed in the step a) additionally performed in each case is at least partially the liquid absorption medium A₃ obtained from the step b) performed immediately beforehand. This means that in particular the water content of the liquid absorption medium A₁ employed in the step a) additionally performed in each case and of the liquid absorption medium A₃ from the step b) performed immediately beforehand is identical.

It is yet more preferable when in this embodiment liquid absorption medium A₂ is supplied with heat from liquid absorption medium A₃. This may be carried out in an additional heat exchanger W_(y1), in particular selected from the group consisting of shell and tube heat exchangers and plate and frame heat exchangers. This makes it possible to carry out the process according to the invention in a particularly energy efficient fashion.

The invention also relates in a further aspect to an apparatus V₂ for dehumidifying a moist gas mixture which in particular comprises CO₂, comprising the components

-   -   (i) a liquid absorption medium A_(1*) which comprises a mixture         of triethylene glycol and at least one salt S selected from the         group consisting of Q⁺A⁻, Q⁺(R¹O)₂PO₂ ⁻, (Q⁺)₂R²OPO₃ ²⁻,         Q⁺M⁺R³OPO₃ ²⁻.     -   (ii) at least one water absorption unit W_(abs2) set up for         contacting the moist gas mixture with the liquid absorption         medium A_(1*),     -   (iii) at least one water desorption unit W_(des2) which         comprises a heat exchanger W_(x2) and is set up for at least         partially removing water from a liquid absorption medium A_(1*),     -   (iv) and a circuit U₂ which connects the water absorption unit         W_(abs2) with the water desorption unit W_(des2) and by means of         which the liquid absorption medium A_(1*) may be circulated,     -   characterized in that     -   Q⁺ is a dialkylimidazolium cation in which the alkyl groups each         independently of one another have 1 to 6 carbon atoms,     -   wherein A⁻ is an anion selected from the group consisting of         acetate, propionate, R′SO₃ ⁻, HSO₄ ⁻, R″SO₄ ⁻, wherein R′, R″         are independently of one another an alkyl group having 1 to 6         carbon atoms,     -   wherein R¹, R², R³ are each independently of one another an         alkyl group having 1 to 6 carbon atoms and M⁺ is an alkali metal         ion, preferably Li⁺, K⁺ or Na⁺, yet more preferably K⁺ or Na⁺,         with the proviso that the salt S is not dimethylimidazolium         methylsulfonate.

The apparatus according to the invention V₂ is suitable for dehumidification of a moist gas mixture, in particular comprising CO₂, preferably comprising CO₂ in a proportion of 1 to 140000 ppm, more preferably 100 ppm to 20000 ppm, yet more preferably 350 ppm to 10000 ppm.

The apparatus V₂ according to the invention comprises the following components:

As a first component the apparatus according to the invention V₂ comprises a liquid absorption medium A_(1*) which comprises a mixture of triethylene glycol and at least one salt S selected from the group consisting of Q⁺A⁻, Q⁺(R¹O)₂PO₂ ⁻, (Q⁺)₂R²OPO₃ ²⁻, Q⁺M⁺R³OPO₃ ²⁻, wherein Q⁺ is a dialkylimidazolium cation in which the alkyl groups each independently of one another have 1 to 6 carbon atoms,

-   wherein A⁻ is an anion selected from the group consisting of     acetate, propionate, R′SO₃ ⁻, HSO₄ ⁻, R″SO₄ ⁻, wherein R′, R″ are     independently of one another an alkyl group having 1 to 6 carbon     atoms, -   wherein R¹, R², R³ are each independently of one another an alkyl     group having 1 to 6 carbon atoms and M+ is an alkali metal ion,     preferably Li⁺, K⁺ or Na⁺, yet more preferably K⁺ or Na⁺, with the     proviso that the salt S is not dimethylimidazolium methylsulfonate.

In a preferred embodiment of the apparatus V₂ according to the invention in A_(1*), Q₊ is a dialkylimidazolium cation in which the alkyl groups each independently of one another have 1 to 4 carbon atoms and R¹, R², R³ are each independently of one another methyl or ethyl and A⁻ is selected from the group consisting of acetate, propionate R′SO₃ ⁻, HSO₄ ⁻, R″SO₄ ⁻, wherein R′, R″ are each independently of one another methyl or ethyl.

In a more preferred embodiment of the apparatus V₂ according to the invention the salt S in A_(1*) is selected from the group consisting of Q⁺A⁻, Q⁺(R¹O)₂PO₂ ⁻, and Q⁺ is a dialkylimidazolium cation in which the alkyl groups are each independently of one another methyl, ethyl, n-propyl, iso-propyl or n-butyl and A⁻ is an anion selected from the group consisting of acetate, propionate, R′SO₃ ⁻, R″SO₄ ⁻, wherein R′, R″ are independently of one another an alkyl group having 1 to 4 carbon atoms, wherein R¹ is methyl or ethyl.

In a more preferred embodiment of the apparatus V₂ according to the invention the salt S in A1* is selected from the group consisting of Q⁺A⁻, Q⁺(R¹O)₂PO₂ ⁻, and Q⁺ is a dialkylimidazolium cation in which the alkyl groups are each independently of one another methyl, ethyl, n-propyl, iso-propyl or n-butyl and A⁻ is an anion selected from the group consisting of acetate, propionate, R′SO₃ ⁻, R″SO₄ ⁻, wherein R′, R″ are independently of one another an alkyl group having 1 to 2 carbon atoms, wherein R¹ is methyl or ethyl.

In a yet more preferred embodiment of the apparatus V₂ according to the invention the salt S in A_(1*) is selected from the group consisting of Q⁺A⁻, Q⁺(R¹O)₂PO₂ ⁻, and Q⁺ is selected from the group consisting of 1,3-dimethylimidazolium cation, 1,3-diethylimidazolium cation, 1-ethyl-3-methylimidazolium cation and A⁻ is an anion selected from the group consisting of acetate, propionate, MeSO₃ ⁻, EtSO₃ ⁻, MeSO₄ ⁻, EtSO₄ ⁻ and R¹ is methyl or ethyl.

In a yet still more preferred embodiment of the apparatus V₂ according to the invention the salt S in A_(1*) is selected from the group consisting of Q⁺A⁻, Q⁺(R¹O)₂PO₂ ⁻, and Q⁺ is selected from the group consisting of 1,3-dimethylimidazolium cation, 1,3-diethylimidazolium cation, 1-ethyl-3-methylimidazolium cation and A⁻ is an anion selected from the group consisting of acetate, propionate, MeSO₃ ⁻, EtSO₄ ⁻, and R¹ is methyl or ethyl, yet more preferably R¹=ethyl.

In a particularly preferred embodiment of the apparatus V₂ according to the invention Q⁺ is selected from the group consisting of 1,3-dimethylimidazolium cation, 1,3-diethylimidazolium cation, 1-ethyl-3-methylimidazolium cation, and A⁻ is an anion selected from the group consisting of acetate, propionate. It is yet still more preferable if Q⁺ is then a 1,3-dimethylimidazolium cation.

The liquid absorption medium A_(1*) may be employed in the form of the pure mixture of triethylene glycol and all salts S. Alternatively and more preferably the liquid absorption medium A_(1*) is an aqueous solution in which the total weight of triethylene glycol and all salts S is preferably at least 70 wt % based on the total weight of the aqueous solution. It is yet more preferable when the total weight of triethylene glycol and all salts S in A_(1*) is at least 80 wt % based on the total weight of the aqueous solution, yet more preferably 85 wt %, yet more preferably 90 wt %, yet still more preferably 99 wt %.

In the liquid absorption medium A_(1*) the ratio of triethylene glycol to the salts S in the absorption medium is not further restricted. However, it is preferable to employ in the apparatus V₂ according to the invention an absorption medium A_(1*) in which the ratio of the total weight of the triethylene glycol to the total weight of all salts S is in the range 1:9 to 9:1, more preferably 2:8 to 8:2, yet more preferably 3:7 to 7:3, yet still more preferably 3:7 to 6:4, yet still more preferably 3:7 to 1:1, most preferably 3:7 to 4:6, with very greatest preference 3:7.

As a second component the apparatus V₂ according to the invention comprises a water absorption unit W_(abs2) set up for contacting the moist gas mixture with the liquid absorption medium A_(1*). The water absorption unit W_(abs2) may in particular comprise an additional heat exchanger W_(z2) set up such that the liquid absorption medium A_(1*) is coolable. Employable water absorption units W_(abs2) of this kind include in particular the water absorbers known to those skilled in the art. Said absorbers are based on the principle of increasing the surface area of the liquid absorption medium A_(1*) and simultaneously achieving the longest possible residence time of the liquid absorption medium A_(1*) in the water absorber during absorption of the water. It is in particular possible here to employ water absorbers selected from the group of: packed beds, spray columns, falling-films, bubble columns, tray columns, wet scrubbers (for example Venturi scrubbers), stirred tanks and combinations of these absorbers. It is particularly preferable to employ falling-films, in particular shell and tube falling-films, as water absorbers.

As a third component the apparatus V₂ according to the invention comprises a water desorption unit W_(des2) which comprises a heat exchanger W_(x2) and is set up for at least partially removing water from the liquid absorption medium A_(1*). Particularly the combinations of heat exchanger and water desorber known to those skilled in the art may be employed therefor. The water desorption unit W_(des2) is based on the principle of supplying heat to the liquid absorption medium A_(1*), increasing the surface area of the liquid absorption medium A_(1*) and simultaneously achieving the longest possible residence time of the liquid absorption medium A_(1*) in the water desorption unit.

Employable water desorption units W_(des2) comprising a heat exchanger W_(x2) include in particular the combinations of heat exchanger and water desorber known to those skilled in the art, in particular horizontal tube evaporators having an upstream heat exchanger, in particular shell and tube heat exchangers, plate and frame heat exchangers. In addition the water desorption unit W_(des2) comprising a heat exchanger W_(x2) may also be a water desorber having an integrated heat exchanger. Such water desorbers having an integrated heat exchanger are in particular climbing film evaporators, long tube vertical evaporators, short tube vertical evaporators, forced circulation evaporators, agitated thin film evaporators. It is particularly preferable to employ as water desorption unit W_(des2) falling-films, in particular shell and tube falling-films.

As a fourth component the apparatus V₂ according to the invention comprises a circuit U₂ which connects the water absorption unit W_(abs2) with the water desorption unit W_(des2) and by means of which the liquid absorption medium A_(1*) may be circulated. The circuit U₂ is preferably a conduit, yet more preferably selected from the group consisting of tube, hose. In a further preferred embodiment the circuit U₂ also comprises a pump.

In a preferred embodiment the apparatus V₂ comprises a further heat exchanger W_(y2) (additional to the heat exchanger W_(x2) comprised in the water desorption unit W_(des2)). The heat exchanger W_(y2) is set up such that liquid absorption medium A_(1*) sent from the water absorption unit W_(abs2) to the water desorption unit W_(des2) may be supplied with heat from liquid absorption medium A_(1*), said medium being conducted away from the water desorption unit W_(des2). This can be ensured in particular by employing as heat exchanger W_(y2) in particular a heat exchanger selected from shell and tube heat exchangers, plate and frame heat exchangers.

In a further preferred embodiment the apparatus V₂ is part of an absorption chiller (synonymous with “absorption heat pump” according to the invention). This absorption chiller then comprises as further components a condenser, an evaporator and a coolant, wherein the coolant is water.

The condenser is in particular connected to the water desorption unit W_(des2) via a conduit and is set up for condensing water at least partially removed from the liquid absorption medium A_(1*) in the water desorption unit W_(des2). The condenser preferably also comprises a cooling water circuit.

The evaporator is in particular connected to the condenser via a conduit (which may comprise a throttling means) and connected to the water absorption unit W_(abs2) via a further conduit and is set up for evaporating condensed water from the condenser. The evaporator preferably also comprises a pressure of <1 bar, more preferably <0.1 bar, to permit evaporation of the condensed water at the lowest possible temperatures. The evaporator may further preferably additionally comprise an apparatus from which heat may be drawn and so that the condensed water may be evaporated (for example a coolant conduit in which coolant is passed into the space in which water is evaporated).

Finally, the present invention also relates to the absorption medium A_(1*) itself, wherein the absorption medium A_(1*) comprises a mixture of triethylene glycol and at least one salt S selected from the group consisting of Q⁺A⁻, Q⁺(R¹O)₂PO₂ ⁻, (Q⁺)₂R²OPO₃ ²⁻, Q⁺M⁺R³OPO₃ ²⁻, wherein Q⁺ is a dialkylimidazolium cation in which the alkyl groups each independently of one another have 1 to 6 carbon atoms,

-   wherein A⁻ is an anion selected from the group consisting of     acetate, propionate, R′SO₃ ⁻, HSO₄ ⁻, R″SO₄ ⁻, wherein R′, R″ are     independently of one another an alkyl group having 1 to 6 carbon     atoms, -   wherein R¹, R², R³ are each independently of one another an alkyl     group having 1 to 6 carbon atoms and M⁺ is an alkali metal ion,     preferably Li⁺, K⁺ or Na⁺, yet more preferably K⁺ or Na⁺, with the     proviso that the salt S is not dimethylimidazolium methylsulfonate.

In a preferred embodiment of the absorption medium A_(1*) according to the invention Q₊ therein is a dialkylimidazolium cation in which the alkyl groups each independently of one another have 1 to 4 carbon atoms and R¹, R², R³ are each independently of one another methyl or ethyl and A⁻ is selected from the group consisting of acetate, propionate R′SO₃ ⁻, HSO₄ ⁻, R″SO₄ ⁻, wherein R′, R″ are each independently of one another methyl or ethyl.

In a more preferred embodiment of the absorption medium A_(1*) according to the invention the salt S is selected from the group consisting of Q⁺A⁻, Q⁺(R¹O)₂PO₂ ⁻, and Q⁺ is a dialkylimidazolium cation in which the alkyl groups are each independently of one another methyl, ethyl, n-propyl, iso-propyl or n-butyl and A⁻ is an anion selected from the group consisting of acetate, propionate, R′SO₃ ⁻, R″SO₄ ⁻, wherein R′, R″ are independently of one another an alkyl group having 1 to 4 carbon atoms, wherein R¹ is methyl or ethyl.

In a more preferred embodiment of the absorption medium A_(1*) according to the invention the salt S therein is selected from the group consisting of Q⁺A⁻, Q⁺(R¹O)₂PO₂ ⁻, and Q⁺ is a dialkylimidazolium cation in which the alkyl groups are each independently of one another methyl, ethyl, n-propyl, iso-propyl or n-butyl and A⁻ is an anion selected from the group consisting of acetate, propionate, R′SO₃ ⁻, R″SO₄ ⁻, wherein R′, R″ are independently of one another an alkyl group having 1 to 2 carbon atoms, wherein R′ is methyl or ethyl.

In a yet more preferred embodiment of the absorption medium A_(1*) according to the invention the salt S therein is selected from the group consisting of Q⁺A⁻, Q⁺(R¹O)₂PO₂ ⁻ and Q⁺ is selected from the group consisting of 1,3-dimethylimidazolium cation, 1,3-diethylimidazolium cation, 1-ethyl-3-methylimidazolium cation, and A⁻ is an anion selected from the group consisting of acetate, propionate, MeSO₃ ⁻, EtSO₃ ⁻, MeSO₄ ⁻, EtSO₄ ⁻, and R¹ is methyl or ethyl.

In a yet still more preferred embodiment of the absorption medium A_(1*) according to the invention the salt S therein is selected from the group consisting of Q⁺A⁻, Q⁺(R¹O)₂PO₂ ⁻ and Q⁺ is selected from the group consisting of 1,3-dimethylimidazolium cation, 1,3-diethylimidazolium cation, 1-ethyl-3-methylimidazolium cation and A⁻ is an anion selected from the group consisting of acetate, propionate, MeSO₃ ⁻, EtSO₄ ⁻, and R¹ is methyl or ethyl, yet more preferably R¹=ethyl.

In a particularly preferred embodiment of the absorption medium A_(1*) according to the invention S therein is a salt having the structure Q⁺A⁻, wherein A⁻ is an anion selected from the group consisting of acetate, propionate.

The liquid absorption medium A_(1*) may be employed in the form of the pure mixture of triethylene glycol and all salts S. Alternatively and more preferably the liquid absorption medium A_(1*) is an aqueous solution in which the total weight of triethylene glycol and all salts S is preferably at least 70 wt % based on the total weight of the aqueous solution. It is yet more preferable when in A_(1*) the total weight of triethylene glycol and all salts S is at least 80 wt % based on the total weight of the aqueous solution, yet more preferably 85 wt %, yet more preferably 90 wt %, yet still more preferably 99 wt %.

In the absorption medium A_(1*) according to the invention the ratio of triethylene glycol to the salts S in the absorption medium is not further restricted. However, in the absorption medium A_(1*) the ratio of the total weight of the triethylene glycol to the total weight of all salts S is preferably in the range 1:9 to 9:1, more preferably 2:8 to 8:2, yet more preferably 3:7 to 7:3, yet still more preferably 3:7 to 6:4, yet still more preferably 3:7 to 1:1, most preferably 3:7 to 4:6, with very greatest preference 3:7.

The FIGS. 1 and 2 elucidated hereinbelow show preferred embodiments of the process according to the invention and the apparatus according to the invention.

FIG. 1 (abbreviated to “FIG. 1”) shows an embodiment of the apparatus V₂/V₁ according to the invention.

The apparatus V₁ shown in FIG. 1 comprises a water absorption unit W_(abs1) <103> (with optional additional heat exchanger W_(z1) <104>) to which a conduit <101> leads and from which a conduit <102> leads away, a water desorption unit W_(des1) which comprises a heat exchanger W_(x1) <108> and a water desorber <109> and to which conduit <111> leads and from which conduits <110>, <112> and <113> lead away, and a circuit U₂ formed from conduits <106>, <111> and <113> or <106>, <111>, <112> and <105> (in each case optionally with conduit <114>). The apparatus in FIG. 1 may also optionally comprise a further heat exchanger W_(y1) <107> to which conduits <106> and <112> lead and from which conduits <105> and <111> lead away. In addition the apparatus V₁ also comprises a liquid absorption medium A₁. Said medium is located in one or more of the abovementioned components water absorption unit W_(abs1), water desorption unit W_(des1), circuit U₁. The water absorption unit W_(abs1) <103> may optionally also comprise an additional heat exchanger W_(z1) <104> with which the liquid absorption medium A₁ may be brought to a temperature of 30° C. to 100° C., preferably 30° C. to 60° C., more preferably 30° C. to 50° C., yet more preferably 40° C. to 50° C. Optionally, circuit U₂ may also additionally comprise a pump for conveying the liquid absorption medium A₁.

The process according to the invention will now be illustratively described with reference to apparatus V₁ using FIG. 1:

A stream of a moist gas mixture G₁ having a temperature of 30° C. to 100° C. (said stream may for example be moist air, moist natural gas or moist gas mixture—see also FIG. 2 with regard to this option) is supplied via conduit <101> to a water absorption unit W_(abs1) <103> and contacted there with the liquid absorption medium A₁ supplied to the water absorption unit W_(abs1) <103> via the conduit <105> or via the conduit <113>. The water absorption unit W_(abs1) <103> may be any of the water absorbers recited hereinabove for W_(abs1), in particular a falling-film. Contacting, in the water absorption unit W_(abs1) <103>, gas mixture G₁ supplied via conduit <101> with the liquid absorption medium A₁ supplied via the conduit <105> or via the conduit <113> affords a liquid absorption medium A₂ having an elevated water content compared to the liquid absorption medium A₁ and a stream of a gas mixture G₂ discharged via the conduit <102>, wherein G₂ has a relatively low water content compared to the moist gas mixture G₁. Depending on the application G₂ is in particular dehumidified natural gas. The water absorption unit W_(abs1) <103> may optionally also comprise an additional heat exchanger W_(z1) <104> with which the liquid absorption medium A₁ may be brought to a temperature of 30° C. to 100° C., preferably 30° C. to 60° C., more preferably 30° C. to 50° C., yet more preferably 40° C. to 50° C. Preferably via the conduits <106>, <111> and the heat exchanger W_(y1) <107> (or, when heat exchanger W_(y1) <107> is not employed, via conduits <106>, <111> and <114>) the liquid absorption medium A₂ is then passed to the water desorption unit W_(des1) composed of the heat exchanger W_(x1) <108> and the water desorber <109>. The water-laden liquid absorption medium A₂ may additionally be supplied with heat in the optional heat exchanger W_(y1) <107>. The at least partial removal of water from the liquid absorption medium A₂ is then carried out in the water desorber <109> to afford a liquid absorption medium A₃ having a relatively low water content compared to the liquid absorption medium A₂. The water removed is then discharged from the water desorber <109> as liquid or vapour, preferably as vapour, via conduit <110>. The liquid absorption medium A₃ is then discharged from the water desorber <109> and returned to the water absorption unit W_(abs1) <103>. This may be carried out directly, i.e. via the conduit <113> which is shown in dashed form in FIG. 1. Alternatively and preferably the liquid absorption medium A₃ may also be supplied via the conduit <112> to the optional heat exchanger W_(y1) <107> in which the liquid absorption medium A₂ supplied via conduit <106> to the optional heat exchanger W_(y1) <107> is then supplied with heat from the liquid absorption medium A₃ supplied via conduit <112> to the optional heat exchanger W_(y1) <107>. Once the concentrated liquid absorption medium A₃ has been supplied to the water absorption unit W_(abs1) via conduit <105> or <113> said medium is reused as A₁ for at least partially dehumidifying the gas stream in a new cycle.

FIG. 2 (abbreviated as “FIG. 2”) shows in schematic fashion an absorption chiller into which an apparatus V₂ is integrated. The constituents <101> to <114> are shown as for the apparatus V₂ described in FIG. 1. Additionally, the absorption chiller in FIG. 2 also comprises a condenser <211> which is connected to the water desorption unit W_(des2) <109> via the conduit <110> and is set up for condensing water at least partially removed from the liquid absorption medium A_(1*) in the water desorption unit W_(des2). Condenser <211> preferably also comprises a heat exchanger <212> with which cooling water may be supplied.

The absorption chiller shown in FIG. 2 also comprises an evaporator <214> connected to the condenser <211> via a conduit <216> (which may optionally comprise a throttling means <213>) and connected via the conduit <101> with the water absorption unit W_(abs2). The evaporator <214> is set up to evaporate condensed water from the condenser. Additionally, the evaporator <214> can further preferably also comprise a heat exchanger <215> which supplies a medium, heat being drawn off from the medium to thus evaporate the condensed water (for example a coolant conduit with, in particular, water as coolant, this coolant being passed into the evaporator <214>). In an embodiment of the process according to the invention (described hereinbelow with reference to apparatus V₁ using FIG. 2) moist gas mixture G originating from evaporator <214> is passed via the conduit <101> to the water absorption unit W_(abs1) <103>. The water removed in water desorption unit W_(des1) is supplied via the conduit <110> to the condenser <211> in which said water is recondensed. A cooling water circuit as heat exchanger <212> installed in the condenser is optionally likewise used therefor. The condensed water is then supplied via a conduit <216> to the evaporator <214> in which the evaporation of water is effected in particular at low pressures, thus bringing about a cooling effect. This may optionally also be effected using a throttling means <213>. This achieves a cooling action in the evaporator <214> and, for example, coolant may be cooled via the heat exchanger <215>. The water vapour generated is then returned to the water absorption unit W_(abs1) <103> via conduit <101>.

The examples which follow are intended to elucidate the present invention without limiting said invention in any way.

EXAMPLES

1. Materials

The ionic liquids used were obtained from Sigma-Aldrich or by procedures described in the prior art (e.g. summarized in DE 10 2014 214 670 A1). The CO₂ content of the gas stream was determined by infrared spectroscopy.

2. Viscosity Measurements

The viscosity of the following ionic liquids (comparative experiments: pure; inventive examples in admixture with 30 wt % of triethylene glycol) was measured according to DIN 54453. The results are shown in Table 1 which follows.

TABLE 1 Mixture with Pure 30 wt % (>99%) of TEG (compar- (inventive Ionic liquid tested ative) examples) 1,3-dimethylimidazolium propionate 46.1 25.8 1-ethyl-3-methylimidazolium diethylphosphate 135.8 36.6 1-ethyl-3-methylimidazolium methylsulfonate 40.5 14.4 1-ethyl-3-methyl-imidazolium acetate 43.0 26.3 1,3-dimethylimidazolium acetate 36.6 24.4 1,3-dimethylimidazolium diethylphosphate 134.1 42.3

It is apparent from the table that for the mixtures comprising 30 wt % of TEG according to the inventive examples a lower viscosity compared to the pure ionic liquid was achieved for all ionic liquids. The mixtures according to the inventive examples are therefore much more suitable for use for instance in dehumidification of gas streams than the pure substances.

3. Experiments Concerning Gas Streams Comprising CO₂

100 mL of the ionic liquids listed in Table 2, in pure form and in admixture with 30 wt % of triethylene glycol respectively, were contacted with a gas stream comprising 140000 ppm of CO₂ in a bubble cell reaction vessel comprising a frit at 40° C. for 1 h. The results observed are summarized in Table 2.

TABLE 2 Mixture with 30 wt % Ionic liquid tested Pure of TEG 1,3-dimethylimidazolium propionate deposit no deposit 1,3-dimethylimidazolium acetate deposit no deposit 1-ethyl-3-methylimidazolium chloride deposit deposit 1,3-dimethylimidazolium chloride deposit deposit 1,3-dimethylimidazolium methyl sulphonate deposit deposit

It is apparent from the results according to table 2, that precipitation of the ionic liquids upon contact with CO₂ was prevented by addition of triethylene glycol only in the case of MMIM Ac and MMIM Prop.

All references cited herein are fully incorporated by reference. Having now fully described the invention, it will be understood by those of skill in the art that the invention may be practiced within a wide and equivalent range of conditions, parameters and the like, without affecting the spirit or scope of the invention. 

What is claimed is:
 1. A process for dehumidifing a moist gas mixture G₁, wherein the moist gas mixture G₁ comprises CO₂, said process comprising: contacting the moist gas mixture G₁ with a liquid absorption medium A₁, wherein A₁ comprises a mixture of triethylene glycol and at least one salt S selected from the group consisting of: Q⁺A⁻; to obtain a gas mixture G₂, which has a lower water content compared to moist gas mixture G₁, and a liquid absorption medium A₂, which has an elevated water content compared to the liquid absorption medium A₁;, wherein: a) Q⁺ is selected from the group consisting of 1,3-dimethylimidazolium cation, 1,3-diethylimidazolium cation, and 1-ethyl-3-methylimidazolium cation; and b) A⁻ is an anion selected from the group consisting of acetate and propionate.
 2. The process of claim 1, wherein the liquid absorption medium A₁ is an aqueous solution in which the total weight of triethylene glycol and all salts S is at least 70 wt % based on the total weight of the aqueous solution.
 3. The process of claim 1, wherein, in the liquid absorption medium A₁, the ratio of the weight of the triethylene glycol to the total weight of all salts S is in the range 1:9 to 9:1.
 4. The process of claim 1, wherein the proportion of CO₂ in the moist gas mixture G₁ is in the range 1 ppm to 140000 ppm.
 5. An absorption medium consisting of a mixture of triethylene glycol and at least one salt S selected from the group consisting of: Q⁺A⁻, wherein: a) Q⁺ is selected from the group consisting of 1,3-dimethylimidazolium cation, 1,3-diethylimidazolium cation, and 1-ethyl-3-methylimidazolium cation; and b) A⁻ is an anion selected from the group consisting of acetate and propionate.
 6. The absorption medium of claim 5, wherein said absorption medium is an aqueous solution in which the total weight of triethylene glycol and all salts S is at least 70 wt % based on the total weight of the aqueous solution.
 7. The absorption medium of claim 5, wherein the ratio of the weight of the triethylene glycol to the total weight of all salts S is in the range 1:9 to 9:1. 